1. Field of the Invention
The present invention relates to coding transmission equipment in which an input signal, such as a voice signal or a picture signal, is coded and transmitted to a receiver side from a transmitter side and a received signal is decoded to become an original signal at the receiver side. In particular, the present invention relates to low bit rate coding equipment (bandwidth compression coding equipment), such as an ADPCM (Adaptive Differential Pulse Code Modulation) system CODEC (COder and DECoder), which carries out low bit rate coding of a voice or picture signal.
When the voice signal, etc., is transmitted, bandwidth compression is required to ensure an efficient utilization of a transmission line. The low bit rat coding transmission equipment for the voice signal carries out information compression while maintaining the quality of the voice signal, which enables a reduction of line costs for the transmission of the voice information in equipment used for, for example, mobile radio communication, satellite communication, or office inter communication system, and a reduction of storage capacity needed, for example, for the storage of voice information in an audio response system.
2. Description of the Related Art
ADPCM coding equipment is related to the present invention in that it carries out coding by adaptive quantization. In the ADPCM coding equipment, a prediction error E corresponding to a difference between a current input signal X and a predicted value X obtained on the basis of past input signals is produced and quantized at the transmitter side. At this time, adaptive quantization, in which the size of a quantizing step is increased or decreased in accordance with a quantizing level of a signal sampled just prior to the current sampled signal, is carried out in order to realize a quick adaptation to a sudden change of the predicted error, instead of uniform quantization, whereby a high quality recovered signal is realized.
In this case, however, the sizes to which the quantizing step can increased or decreased are limited, and therefore, when the input signals are voice signals, for example, a voiced sound, an unvoiced sound, or a voicelessness, etc., the nature of the signals changes from moment to moment and the coding system of the prior art cannot cope with these rapid variations in the voice signal. As a result, an optimum quantization is not realized, and thus the voice recovered at the receiver side has a poor quality.